In a driving force transmission path from an engine of a wheel loader to wheels (tires), there provided a transfer, a modulation clutch, a torque converter having lockup clutch (torque converter, lockup clutch), a transmission having a forward-drive clutch, a backward-drive clutch and speed stage clutches, and a reduction gear.
An output (torque) of the engine is transmitted to the wheels (tires) through the transfer, the modulation clutch, the lockup clutch or torque converter in the torque converter having lockup clutch, the transmission, and the reduction gear.
Conventionally, the modulation clutch is provided in the working vehicle such as wheel loader between the engine and the torque converter for changing the degree of engagement between an input side and an output side to vary the power transmitted from the engine to the torque converter. The modulation clutch is formed by a hydraulic clutch, and the hydraulic clutch performs engagement operation (connection operation) or releasing operation (disconnection operation) by changing the amount of hydraulic pressure.
The degree of engagement of the modulation clutch is adjusted by an inching pedal. By changing, as required, the driving force transmitted to the tires by the modulation clutch, it becomes possible to implement the inching operation, namely, the creeping-speed travel or appropriate combined operation between work equipment and travel, whereby working efficiency can be improved.
The torque converter having lockup clutch is disposed between the modulation clutch and the transmission. When a load is applied to the wheels, that is, when a force that prevents rolling of the wheels is applied, the torque converter in the torque converter having lockup clutch automatically and steplessly changes the torque transmitted to the transmission in accordance with change in the load, and changes the driving force of the wheels to prevent the engine stall. Note that, in order to ensure the tractive force during operation, the torque converter of the wheel loader has a relatively large stall-torque ratio as compared with other working vehicles.
The lockup clutch in the torque converter having lockup clutch is located between the modulation clutch and the transmission, and is provided in parallel with the torque converter in the torque converter having lockup clutch. For covering up the power loss in the torque converter, the lockup clutch becomes a lockup state (clutch engagement state) by directly coupling a pump with a turbine of the converter in a speed range where the property of the torque converter is not required.
In the wheel loader, the lockup clutch is automatically operated in accordance with conditions. This means, the lockup clutch is automatically made ON (engagement operation) and OFF (releasing operation) in accordance with conditions.
The transmission selectively implements the engagement operation or the releasing operation of the forward-drive clutch, a backward-drive clutch and speed stage clutches in accordance with a direction to be advanced, a driving force required, and a speed (traveling speed) required.
A cab of the wheel loader is provided with a forward-backward selecting operation lever, a speed changing lever and a kickdown switch. By operating the forward-backward selecting operation lever or the speed changing lever, a shift range including respective speed stages of a forward drive (F), a backward drive (R), a first speed, a second speed, a third speed, and a fourth speed can be selected.
When the forward-backward selecting operation lever and the speed changing lever are operated to be positions of “forward” and “second speed,” the forward-drive clutch and the second speed stage clutch are selectively engaged in the transmission. The engagement is fixed to this forward second speed stage (referred to as “F2”), as long as the speed changing lever is not operated to be other position or the kickdown switch is not operated to be ON.
The kickdown switch is provided, for example, at a work equipment operation lever. When the forward-backward selecting operation lever and the speed changing lever are operated to be positions of “forward” and “second speed” while the kickdown switch is operated to be ON, a downshift is made to be a forward first speed stage (referred to as “F1”). As a result, further strong driving force (tractive force) can be obtained at the wheels. That is, the position of “second speed” has a shift range including the second speed and the first speed (at the time when the kickdown switch is ON).
Note that, at the position of “forth speed,” the speed change is automatically performed between the second speed, the third speed, and the fourth speed. By operating the kickdown switch to be ON, the downshift is made from the fourth speed to the third speed. Additionally, at the position of “third speed,” the speed change is automatically performed between the second speed and the third speed. By operating the kickdown switch to be ON, the speed is shifted down from the third speed to the second speed.
As one typical operation mode performed by the wheel loaders, there exists a V-shape (loading) operation.
The V-shape operation is an operation in which the wheel loader advances into a mound of soil and excavates the soil; the wheel loader reverses after excavating the soil; the wheel loader changes its direction upon arriving at a direction changing point; the wheel loader goes forward and loads the soil into a hopper or a dump truck; and, the wheel loader repeats this path.
At the time of excavation, the wheel loader performs an operation in which the work equipment for excavation is entered toward the mound of soil while accelerating. At this time, the load applied to the wheels is heavy, and hence a large driving force from the engine is required at the wheels.
Since, during the V-shape operation, the most of the work is operated at a low speed and requires the large driving force (tractive force), the work is usually operated by maintaining the speed changing lever to be the “second speed.” Additionally, when the wheel loader operates extremely heavy excavating operation, such as when entered into the mound of soil, the kickdown switch is operated to be ON on the basis of operator's will, and the speed stage is shifted down to the forward first speed stage (F1).
Additionally, during the V-shape operation, forward-backward changing is repeatedly performed at a lower speed, and the wheel loader mainly operates its (heavy) excavating work in a situation where the vehicle speed is almost zero or close to zero. Therefore, in order to prevent the engine stall, the lockup clutch is usually in a released state, and the driving force of the engine is transmitted to the wheels through the torque converter (hereinafter, referred to as torque converter operation state).
As described above, during the V-shaped operation, the “torque converter operation state” is always maintained. When the larger driving force is necessary at the time of heavy excavation and so on, the speed stage is changed from the forward second speed stage F2 to the forward first speed stage F1. Note that the state where the lockup clutch is in an engaged state is hereinafter referred to as a lockup state.
Note that, at the position of “second speed,” as the vehicle speed decreases, the state sequentially changes in the order of the lockup state with F2, the torque converter operation state with F2, and the torque converter operation state with F1.
Incidentally, the original purpose of using the modulation clutch is to perform the inching operation as described above.
Conventionally, during operation using the wheel loader, tire slips are prevented by controlling the degree of engagement of the modulation clutch.
Patent literature 1 describes an invention in which a difference in the number of rotations between the right and the left driving wheels of the working vehicle is calculated; as the calculated difference in the number of rotations increases, the modulation clutch pressure is decreased to weaken the degree of engagement of the modulation clutch; and the driving force transmitted to the tires is decreased.
Patent Literature: Japanese Patent Application Laid-open No. 2001-146928